Q. My sister-in-law can drink coffee after dinner and sleep like a log. Why doesn't the caffeine keep her up like it does normal people?

My sister-in-law can drink coffee after dinner and sleep like a log. Why doesn't the caffeine keep her up like it does normal people?

"People actually build up a tolerance," says Ruth Frechman, RD, a spokesperson for the American Dietetic Association.

"The more you drink over time, the less it stimulates you." Age, weight, and even your genes can make you more or less susceptible to caffeine's effects as well. Caffeine tolerance is not yet well understood, so it's important to take cues from your own body--not your friends' or family's.